Center pivot irrigation systems, of which Zybach U.S. Pat. No. 2,941,727 is a relatively early example, are widely used for irrigation of large agricultural areas. The lengths of commercial systems of this kind range from about 200 feet to over 1800 feet. In any such system, because the central portion of the radially extending main conduit moves much more slowly than the outer portion, it is necessary and customary to provide for a substantially larger discharge of water at the outer end of the system than at the inner end. Typically, this control is exercised by a gradation of the orifice sizes for the sprinklers or discharge nozzles, using small discharge orifices at the inner portion of the system and substantially larger orifices at the outer end. Another water balance technique that has been applied to basic center pivot irrigation systems entails programmed time control for discharge nozzles of uniform size; a water balance control of this particular kind is disclosed in Chapman U.S. Pat. No. 3,901,422.
The conventional center pivot irrigation system covers a circular area; in a rectangular field, the corners are not irrigated. For a number of years, it has been customary to irrigate a part of the corner areas by means of a large high volume long distance discharge nozzle called an end gun. The end gun is maintained inoperative as the system sweeps past those parts of the field where the arc described by the outer end of the pivoting main conduit approaches relatively close to the edges of the field. The end gun is operated for limited arcuate segments of movement of the system that are aligned with the corners. The end gun is usually turned on and off by a relatively simple circular cam mounted at the central pivot, with adjustable cam members to determine the arcuate limitations for the system at which the end gun is actuated.
More recently, center pivot irrigation systems have been provided with an auxiliary conduit that is pivotally mounted to the outer end of the main conduit. The auxiliary conduit, sometimes called a boom, is held at an angle of close to 90.degree. to the main conduit and is maintained essentially inactive during those intervals in which the outer end of the main conduit is moving closely adjacent the edge of the area to be irrigated. In irrigating the corners of a rectangular field, or in irrigating other areas, in which a part of the area is located beyond the arc described by the end of the main conduit, the auxiliary conduit is pivoted outwardly to an angle of greater than 90.degree. and water is discharged through the auxiliary conduit to irrigate an additional area beyond the boundary of the area that would normally be covered by the system. An early example of this more recent type of center pivot irrigation system is described in Seckler et al. U.S. Pat. No. 3,802,627. Controls for directing the movements of the auxiliary conduit or boom are described in Kircher et al. U.S. Pat. No. 3,797,517 and in Daugherty et al U.S. Pat. No. 3,902,668. The Daugherty et al patent is particularly advantageous, as regards a steering system for the auxiliary conduit boom, because it allows for ready and effective avoidance of obstructions and for the irrigation of peripheral areas of widely varying configuration.
The addition of a pivotal auxiliary conduit at the outer end of a conventional center pivot irrigation system, however, introduces substantial problems with respect to maintenance of water balance in the areas irrigated both by the main conduit and the auxiliary conduit. In particular, the area covered by the auxiliary conduit as it swings outwardly from a first angular alignment to a second angular alignment relative to the end of the boom is substantially different from the area that must be irrigated by the same auxiliary conduit as it swings inwardly through the same angular distance. Furthermore, the differences between the two areas vary in dependence upon the direction in which the main conduit rotates, as compared with the direction in which the auxiliary conduit extends from the main conduit. Finally, with an auxiliary conduit of any substantial length, an increased discharge from the auxiliary conduit produces a reduced discharge from the main conduit as the result of the pressure/discharge characteristics of the pumps ordinarily used to supply water to systems of this general kind, necessitating a system speed adjustment to maintain adequate water balance in the area covered by the main conduit.